1. Field of the Invention
The present invention relates to velocity measurement systems and, more particularly, to such systems applying correlation methods.
2. Description of the Related Technology
There is a class of systems that measure distance or velocity with sound waves. These systems include underwater sonar and medical imaging equipment (hereinafter generally referred to as "sonar systems"). Underwater sonar systems, in particular, may be manifested as current profilers, velocity logs, and so forth.
A current profiler is a type of sonar system that is used to remotely measure water velocity over varying ranges. Current profilers are used in freshwater environments such as rivers, lakes and estuaries, as well as in saltwater environments such as the ocean, for studying the effects of current velocities. The measurement of accurate current velocities is important in such diverse fields as weather prediction, biological studies of nutrients, environmental studies of sewage dispersion, and commercial exploration for natural resources, including oil.
Typically, current profilers are used to measure current velocities in a vertical column of water for each depth "cell" of water up to a maximum range, thus producing a "profile" of water velocities. The general profiler system includes a transducer to generate pulses of sound (which when downconverted to human hearing frequencies sound like "pings") that backscatter as echoes from plankton, small particles, and small-scale inhomogeneities in the water.
Two important types of sonar technology that may be used for current profiling are Doppler and correlation. The basic concept of velocity measurement using signal correlation has been known for many years. For example, in 1964, Dickey, Jr. was issued U.S. Pat. No. 3,147,477 ("Dickey '477") disclosing a system to measure speed including a source of wave energy (or transmitted signal) directed toward a body, multiple receiving devices having a known separation for receiving echoes from the body, correlating means including time delay means, and speed determining means dependent on the quotient of the receiver separation and the time delay. Although the patent document describes an embodiment of a correlation system for an aircraft using radar (electromagnetic signals), it suggests a correlation system for a water-going vessel using sonar (sound signals).
A correlation system inherently requires that the transmitted signal must have some similarity with itself to achieve a maximum correlation at the specified time delay. A correlation system was disclosed by Roeder, et al. (U.S. Pat. No. 4,103,302) wherein signal similarity was achieved by generating a pulse train, or series of repeating pulses. According to the disclosure, the use of repeating pulses provides advantages over the continuous wave (CW) system described in Dickey '477 including: a simplified transmission source, insensitivity to variations in attitude of the conveying vehicle, and improved accuracy and reliability derived from digital electronics.
U.S. Pat. No. 4,244,026 to Dickey, Jr., ("Dickey '026") hereby incorporated by reference, discusses a correlation system using a pulse train from which complex correlation values are employed in velocity measurement. According to Dickey '026, the method of signal processing the complex correlation values estimates components of location by specifically requiring the steps of processing the correlation values as samples of a continuous function of position and curve fitting the samples to the magnitude or real part of the continuous, spacial correlation function to provide an estimate of the peak value of correlation magnitude. Thereafter, the location components at the correlation peak are divided by twice the pulse repetition period of the pulse train to obtain velocity measurements. This signal processing is described in the specification of Dickey '026 at col. 13, lines 7-40.
As previously noted, correlation sonar technology can be used in current profilers. In designing a current profiler, in particular, trade-offs are made among a variety of factors, including maximum profiling range and temporal, spacial (the size of the depth cell), and velocity resolution. Temporal resolution refers to the time required to achieve a velocity estimate with the required degree of accuracy. In typical applications, a current profiler will make a series of measurements which are then averaged together to produce a single velocity estimate with an acceptable level of velocity variance, or squared error.
Over the past ten years, acoustic Doppler technology has been successfully developed for short- and medium-range current profiling applications such as measuring sediment movement in rivers and estuaries. To achieve the longer ranges necessary to study water velocities at deep ocean depths, lower frequencies must be used so that energy losses due to sound absorption by the water can be kept at manageable levels. Although there is no fundamental limitation in the Doppler technology that prevents the use of lower frequencies, there is a practical constraint due to the large size of the acoustic transducers required for these lower frequencies. Existing correlation sonar techniques measure current velocities by transmitting two short pulses from a relatively smaller transducer along a correspondingly wider angle acoustic beam. By careful cross-correlation of spatially sampled returns it is possible to determine the displacement of the acoustic targets within the beam from one pulse to the next, and hence, compute their average velocity. The primary advantage of a correlation sonar is that for comparable sized systems, the correlation sonar is able to utilize a lower frequency regime and, therefore, potentially gains substantial current profiling range.
Nonetheless, it is only recently that the computational burden required of sophisticated signal processing methods, required for accurate correlation processing, has been made feasible by advances in the semiconductor and computer architecture fields. The improved signal processing should include a means for bottom tracking so that the correlation sonar can also serve as a navigation device. It would also be desirable to allow individual users the flexibility of selectably trading off standard deviation and profiling resolution so as to maximize range.